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Phase as it applies to microphone preamps!

Q: “Here is a question to consider for Question of the Day. I’ve never really understood the concept of “phase” and how it applies to mic preamps. My high end mic preamps all have an “invert phase” switch or a symbol indicating 180 degrees. (Some of the lower end pres don’t seem to have this feature.) I’ve tried different settings on different test tracks and have not been able to discern the difference. Can you help? (name withheld in case this concept is so simple I’d be called ”dumb#$$!”)”

A: Hey, just because your name is withheld doesn’t mean I can’t call you dumb#$$! But seriously folks, we’re here to answer these questions so by all means do send them in.

There are a couple of issues to address here. First we should make the distinction between phase and polarity. The switch that you are referring to should be called a polarity switch. Though it is commonly called a phase switch (this is an industry wide problem, not something you got wrong), really all it does is reverse the polarity of the signal.

Understand that in either case we are always talking about how two or more signals relate to one another. Phase and polarity are both relative. When we talk about phase we are referring to timing issues. If two identical waveforms are split (as in with a Y cable) and then one is delayed by a couple of milliseconds it is said to be out of phase. How far signals are out of phase with each other is generally measured in degrees (1 to 359 degrees). When you reverse the polarity of a signal, which is easily accomplished by reversing the two conducting wires, the effect is similar to what happens if you throw it out of phase 180 degrees. At 180 degrees out of phase the signals are completely opposite one another in terms of polarity – while one may be at its positive peak, the other will be at its negative peak, and so on. The difference is that if it is done through phase, one signal is delayed with respect to the other, whereas when it’s done by swapping the conducting wires of one of them, then the polarity is simply reversed. This is what happens when you engage the “phase” switch in your mixer or preamp – it simply reverses the positive and negative wires, which inverts the signal. Sonically the results are nearly the same whether it’s 180 degrees out of phase or swapped polarity. It’s just important to understand what’s really going on.

It’s easy to hear how things sound when the polarity is changed. Try swapping the speaker cable connections at only one speaker of a stereo pair. Instead of red to red and black to black, hook up the cable to speaker as red to black and black to red. The speakers will now act exactly the opposite of each other – when one woofer is moving forward, pushing low frequencies into the room (positive excursion), the other woofer will be moving backward (negative excursion), pulling low frequencies out of the room. Put a mono signal into both and listen to the results (especially noticeable on low frequency material when the speakers are positioned very close together). You can hear how the low frequencies nearly cancel out. Similar experiments can be done on your mixing board and/or preamps as well. It’s usually not as noticeable on a single instrument miked with a single mic (though some engineers argue its importance – see WFTD Absolute Phase). Put two mics up on an acoustic guitar, then reverse the polarity of one of them and you’ll hear a huge difference.

Engineers use this to their advantage in a variety of situations. It can be an easy way to create a very wide stereo image. It’s useful in several stereo miking applications. Be careful though: if your mix will ever be listened to in mono it could end up sounding terrible, with wide panned reversed polarity sounds completely disappearing, so be sure to check it. One of the most common uses is for miking a drum from both sides, such as snare. Top and bottom snare mics can sound great when blended together properly in a mix. But think about what the mic hears. When the head is initially struck it moves toward the bottom mic and away from the top mic. When these two signals get combined, one is a positive signal and the other a negative with each other and the low end will disappear. The solution is to reverse the polarity on one of the mics so that the signals are again in correct polarity. It is usually the bottom mic that is reversed since most of the other mics on a kit are oriented in a downward direction like the top snare mic – this keeps the polarity correct or “in phase.” But as always, listen to the result, swapping the polarity, to make sure it sounds correct.

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